Understanding Mattress Firmness

After processing over 1.15 million mattresses, I've had thousands of conversations with people confused about firmness. They bought "medium-firm" but it felt nothing like the last medium-firm they owned. They followed the "firm is better for your back" advice and woke up in more pain. They read reviews from people who loved a mattress and hated it themselves.

Here's the problem: there's no standardized firmness scale. That "medium-firm" at one store might feel completely different from another brand's "medium-firm." Every manufacturer makes up their own definitions.

This guide covers: the 1-10 scale and why it's unreliable, ILD ratings that actually measure something real, why firmness and support are completely different things, and how your body weight and sleep position determine what you actually need.

Firmness matters whether you're shopping for a standard or odd-size mattress, an RV mattress, or just trying to understand what's inside your mattress. The principles are the same—but the confusion is everywhere.

The 1-10 Scale Is Just a Starting Point

The industry uses a 1-10 firmness scale. A 1 is extra-soft (imagine sinking into a cotton ball). A 10 is extra-firm (essentially concrete). In practice, you'll never find mattresses rated 1-2 or 9-10 because they'd be either unsleepable or painful. Everything clusters between 3 and 8.

Here's how it roughly breaks down:

The industry considers 6.5 the sweet spot that theoretically works for most sleepers. That's where the problems start.

From the facility: When I tear open mattresses labeled "medium-firm," the actual construction varies wildly. Same label, completely different feel. The scale is a rough starting point, nothing more.

ILD Ratings: The Number That Actually Means Something

While firmness scales are subjective, the foam industry does have an objective measurement: Indentation Load Deflection (ILD). This tells you how many pounds of force are required to compress a 4-inch foam sample by 25%. Higher ILD means firmer foam.

The practical ranges:

Most people sleep comfortably on mattresses with comfort layers in the 25-35 ILD range.

But ILD alone doesn't predict how a mattress feels. A complete mattress stacks multiple layers with different ILD ratings—softer on top for pressure relief, firmer beneath for support. The cover fabric, layer thickness, and how materials interact all affect what you actually feel. Two mattresses with identical ILD comfort layers can feel entirely different based on what's underneath.

Different layers with different ILD ratings stacked together — what you feel depends on how they interact, not just the top layer.

Temperature Complicates Things

Memory foam is viscoelastic—it softens with heat and firms up when cold. The same mattress can feel noticeably firmer in January than August. That's not a defect. That's physics.

Firmness and Support Are Not the Same Thing

This is the biggest misconception I encounter. People think firmness equals support. They're completely separate qualities determined by different parts of the mattress.

Firmness describes how the surface feels when you lie down. It's the sensation of the comfort layers.

Support describes whether the mattress maintains your spine's natural alignment. That's the job of the support core beneath those comfort layers.

A properly constructed soft mattress can provide excellent support. The comfort layers cushion your pressure points while the dense support core—high-density polyfoam, coils, or Dunlop latex—prevents your hips from sinking into a hammock shape. Meanwhile, a cheap firm mattress with a weak support core can leave your spine misaligned despite feeling hard as a board.

This matters because a study published in The Lancet followed 313 patients with chronic low back pain. Those on medium-firm mattresses were twice as likely to report improvement compared to those on firm mattresses. The firm-mattress group also used more pain medication.

When a mattress is too firm for your body, it doesn't let your shoulders and hips sink appropriately. This creates pressure points and actually pushes your spine out of alignment—the opposite of what people expect. The myth persists partly because "orthopedic mattress" has become an unregulated marketing term with no standardized meaning.

Your Body Weight Changes Everything

The same mattress genuinely feels different to people of different weights. This isn't perception—it's physics.

Someone weighing 130 pounds will barely compress the comfort layers and may find a "medium" mattress feels firm. Someone weighing 230 pounds will sink deeper into those same layers, potentially reaching the firmer support core, and perceive that mattress as softer.

Under 130 pounds

Softer mattresses in the 3-5 range typically work best. Without sufficient body weight to compress firmer foams, lightweight sleepers end up floating on top and experiencing pressure points rather than relief.

130-230 pounds

The most flexibility. Generally comfortable across the medium range (4-7) depending on sleep position. This is what most mattresses are designed for, which is why "medium-firm" works for many people here.

Over 230 pounds

Typically need firmness in the 6-8 range with higher-density foams and thicker profiles. Without adequate resistance, heavier bodies sink too far, the midsection sags, and the spine goes out of alignment.

What too-soft looks like: This foam wasn't defective — it was simply too soft for the sleeper's weight. Years of compression beyond its design capacity left permanent damage.

Couples with significantly different weights face a real challenge. Solutions include split-firmness mattresses, split king configurations with two separate Twin XL mattresses, or adjustable air mattresses where each side can be independently adjusted.

Sleep Position Determines Where You Need Relief

Your sleep position dictates which parts of your body need cushioning and which need resistance.

Testing how the mattress responds to different body positions — where you sink and where you're supported matters more than overall firmness.

Side Sleepers (3-6 range)

Need the softest mattresses. You're placing concentrated pressure on narrow surfaces—shoulders and hips. Research on sleep position and spinal alignment shows that without adequate cushioning, you develop pain and numbness. The shoulder and hip need to sink appropriately while keeping the spine horizontally aligned.

Back Sleepers (5-7 range)

Need medium to medium-firm. The critical concern is your lumbar region. The mattress must gently fill the natural gap between your lower back curve and the surface without pushing the spine up or letting it sag. Too soft and the hips sink deeper than the shoulders. Too firm and the lower back floats unsupported.

Stomach Sleepers (6-8 range)

The most restricted. Your pelvis is the heaviest part of your body when face-down. If it sinks, the lower spine hyperextends into an unnatural arch. Soft mattresses cause significant back pain for stomach sleepers because there's no cushioning benefit that can offset the spinal misalignment.

Combination Sleepers

Need medium firmness with high responsiveness—the ability to quickly adapt as you move. Hybrid mattresses with pocketed coils or latex excel here. They provide both cushioning and bounce. Slow-responding memory foam can create a "stuck" feeling that makes position changes difficult.

Quick Reference: Firmness by Weight and Position

The Mistakes I See Most Often

Buying too firm

Based on the outdated belief that firm is better for backs. The research doesn't support this. I still find mattresses that show virtually no body impressions because their owners were too light or slept in positions that never properly engaged with the surface.

Buying too soft

And sinking through the comfort layer to the support core. Soft mattresses are among the most returned in the industry, and they deteriorate fastest. When comfort layers are overwhelmed, you end up lying on dense foam or coils never designed for direct contact.

Ignoring body weight

People read reviews from others with completely different bodies and expect similar experiences. A review calling a mattress "perfect medium-firm" might be from someone 100 pounds lighter or heavier than you. That perception is useless for your decision.

Trusting short in-store tests

A few minutes on a showroom floor bears no relationship to hours of sleep. Research shows it takes at least 15 minutes for muscles to relax enough to reveal true comfort issues. Even then, store mattresses have been compressed by hundreds of previous testers. This is why sleep trials exist—your body needs 30-90 days to reveal whether the firmness actually works.

Assuming your preference is permanent

It changes with age, weight fluctuation, injuries, and health conditions. What works at 30 may cause pain at 50. Older adults often need softer mattresses as pressure sensitivity increases. Weight gain may require increased firmness to maintain support.

New Mattresses Feel Different Than Broken-In Ones

Every new mattress exhibits "false firmness"—temporary stiffness from fresh foam cells, tight fabric, and materials that haven't been exposed to air and pressure. If your mattress arrives compressed in a box, it needs 24-72 hours just to expand fully.

Break-in periods vary by material:

During this time, foam cells relax, stitching loosens slightly, and materials begin conforming to your typical sleep positions. Judging a mattress in the first week is premature.

Normal softening continues throughout a mattress's life. All foams gradually break down under daily compression. That's expected. What matters is whether softening becomes excessive.

How Long Different Materials Hold Their Firmness

Material type dramatically affects longevity. Here's what I've observed:

Latex — Still Bouncy After 15+ Years

Low-Density Foam — Collapsed After 4 Years

In nearly every mattress I process, the comfort layers fail first. The support cores usually outlive them by years.

The Marketing BS to Ignore

"Universal comfort" or "perfect for everyone"

No mattress firmness works universally because bodies vary too much. These phrases mean "medium to medium-firm, designed for the largest possible market segment." That's a business strategy, not a sleep solution.

Unregulated firmness terms

"Luxury firm," "plush firm," "relaxed firm" have no standardized definitions. Brands invent these labels for differentiation, not clarity. Even basic terms—soft, medium, firm—mean different things to different manufacturers.

Consumer reviews about firmness

Perception varies dramatically with body weight, previous mattress, sleep position, and individual sensitivity. Someone's "perfect medium" might be your "way too soft." The only reliable test is your own body over multiple nights.

Different Materials Feel Different Beyond Just Firmness

Memory Foam — Slow Conform, "Hugging" Feel

Hybrid — Bounce + Support From Coils

Memory foam conforms slowly, responds to body heat, creates a "hugging" sensation. Can feel warmer. Position changes take more effort.

Latex bounces back immediately, maintains consistent feel regardless of temperature, feels more buoyant. Easier to move around on.

Innersprings and hybrids provide more pushback and bounce. Firmness affected by coil gauge (lower gauge = thicker wire = firmer), coil count, and comfort layer depth above the coils.

For a deeper comparison, see my guide on memory foam vs latex.

How to Actually Find Your Firmness

Start with your body weight and sleep position as primary guides. Under 130 pounds and side sleeping? Begin soft. Over 230 pounds and stomach sleeping? Begin firm. Adjust based on pain or pressure issues.

Ask yourself these questions:

• Do you wake with pain that fades after moving around? That's likely mattress-related.
• Do you sleep better in hotels or other beds? Note whether those beds are firmer or softer.
• Do your shoulders or hips feel pressed? You may need something softer.
• Does your lower back ache? You may need more support—which isn't necessarily firmer but rather better core construction.

Quick test: Press into the mattress with your hand. Quality construction pushes back evenly. If your hand sinks with no resistance, the comfort layer is too soft. If it barely compresses, too firm for most sleepers.

The hand test

Lie on your back and try sliding your hand under your lower back. If it slides easily, the mattress is probably too firm. If you can't slide it at all, probably too soft. Moderate resistance suggests reasonable fit.

Use sleep trials

Insist on at least 90 nights. Honor the break-in period before judging. Keep a simple sleep journal noting pain levels and rest quality. Test all positions. Use your normal pillow and sheets. After 30+ days, evaluate spinal alignment, pressure relief, temperature, and whether you wake rested.

The Bottom Line

What strikes me most after processing over a million mattresses is how many failed not from defects but from mismatch—wrong firmness for the sleeper's body and position.

Tim Sumerfield

Owner, A Bedder World

The "best" mattress doesn't exist as an absolute. Only the best mattress for your specific weight, your specific sleep position, and your specific comfort needs. Ignore universal-comfort marketing. Treat firmness scales as rough starting points. Understand that ILD ratings tell you about foam layers, not finished mattress feel. Weight matters more than most people realize. Sleep position dictates where you need cushioning versus support.

Use sleep trials as the real test. Your body knows what it needs after several weeks of actual sleep—no showroom visit or review can substitute. And remember that your needs will change over time. The firmness that works now may not work in ten years.